The present invention relates generally to an exposure apparatus, and more particularly to an exposure apparatus used to expose a substrate such as a wafer for a semiconductor device, and a glass plate for a liquid crystal device. The present invention is suitable, for example, for a so-called immersion exposure apparatus that immerses in the liquid the final surface of the projection optical system and the surface of the substrate, and exposes the substrate via the liquid.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern of a reticle (mask) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device in the photolithography technology.
The minimum critical dimension (“CD”) transferable by the reduction projection exposure apparatus or resolution is proportionate to the wavelength of the light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with the recent demands for finer semiconductor devices, use of the UV light with a shorter wavelength has been promoted from an KrF excimer laser (with a wavelength of approximately 248 nm) to an ArF excimer laser (with a wavelength of approximately 193 nm).
In this setting, the immersion exposure is one attractive technology to improve the resolution while using a light source, such as the ArF excimer laser. The immersion exposure fills a space between the final surface of the projection optical system and the wafer with the liquid, shortening the effective wavelength of the exposure light, increasing the apparent NA of the projection optical system, and improving the resolution. Since the NA of the projection optical system is defined as NA=n·sin θ, where n is a refractive index of the (liquid) material, the NA increases up to “n” when the filled material has a refractive index greater than that of the air (n>1).
In the immersion exposure, there are roughly two methods of filling the liquid in the space between the final surface of the projection optical system and the wafer: The first method is a method of sinking the final surface of the projection optical system and the entire wafer in a bath. The second method is a local fill method that flows the liquid only in the space between the projection optical system and the wafer surface. Some exposure apparatuses are proposed which use the local fill method. See, for example, Japanese Patent Applications, Publication Nos. 2005-5713, 2005-129810, 2005-286286, and 2005-203681.
The immersion exposure resolves a CD (half pitch) between 40 nm and 60 nm, and the particles floating in the liquid harm the pattern formation. For instance, the particle that adheres to the wafer surface would break the wire structure, and the particle that floats above the wafer surface would partially block the light from the projection optical system, causing the low contrast part.
The particles mixed in the liquid mainly originate from abrasive, such as cerium oxide and iron oxide, and a pitch material, such as asphalt, which are used to polish the lens surface of the final lens in the projection optical system closest to the wafer. These materials adhere to an uneven edge face or side surface of the final lens, dry and fix on the edge face of the final lens due to the storage condition from the polishing process to the cleansing process. The cleansing process that combines alkali cleansing and fluoric acid cleansing cannot remove the particles, such as the abrasive and the pitch material, which are fixed on the edge face of the final lens. When the optical lens having the particles adhered edge face are incorporated into the exposure apparatus, the particles separates from the edge face and floats in the liquid due to the liquid action between the edge face and the liquid supply/recovery tube near the edge face. Since it is tardier to exchange the liquid that exists between the supply/recovery tube and the edge face than to exchange the liquid that exists between the final lens and the wafer, the particles floating in the liquid that exists between the supply/recovery tube near the edge face are likely to agglutinate into a large growing lump.
In addition, the surface of the supply/recovery tube also contacts the liquid, the particle that adheres to the surface can separate and float in the liquid.
Moreover, a top-flatted plate (auxiliary plate) level with the top surface of the wafer also contacts the liquid, and the particle that adheres to the surface is likely to separate and float in the liquid as discussed above.
The particles mixed in the liquid contain a particle caused by a agglutination due to a chemical reaction of the resist applied onto the wafer, and a particle that is generated at the wafer edge while the wafer is being fed to the exposure apparatus.